A variety of processes have been marketed and/or proposed over the years for welding together overlapping portions of a tensioned loop of thermoplastic strap encircling an article. One process employs a heated member to melt a surface layer of each of the strap portions which are then pressed together while the layers merge and cool to form a solidified weld.
In a different process, the strap portions are pressed together, and a layer of each strap portion at the interface is melted by means of ultrasonic energy. The layers then cool and solidify while the overlapping strap portions remain pressed together.
Another process is effected by first pressing the overlapping strap portions together and then creating a unidirectional or multidirectional bodily sliding frictional movement between the contacting surfaces of the overlapping strap portions so as to melt the interface region of the overlapping strap portions. The melted interface region is allowed to solidify at rest, but under pressure, so as to bond the overlapping strap portions together.
The last discussed process, which can be generally designated as friction-fusion welding or friction welding, has proven to be especially effective with conventional thermoplastic strap materials such as nylon, polyester, and polypropylene Such conventional strap is typically provided commercially in widths ranging from about 5 mm. to about 13 mm. and in thicknesses ranging between about 0.25 mm. and about 0.89 mm.
Conventionally produced welded joints in thermoplastic strap have found wide commercial acceptance in many applications. However, a conventional welded joint is typically the weakest part of a tensioned strap loop secured about a package or other object. There is a continuing need for methods and apparatus capable of producing routinely and consistently, and in an economic manner, a welded joint that has greater strength than conventional welded joints in various types of strap. Specifically, it is desired to produce a welded joint that has a strength that approaches, as close as possible, the tensile strength of the strap.
One aspect of the present invention is the discovery that, with certain types of strap, a welded joint can be produced with a unique internal configuration that provides an improved joint with greater strength.
The above-discussed techniques for producing a welded joint in overlapping strap portions typically employ apparatus for continuing to press the strap portions together under pressure while the melted interface layers of the straps cool and solidify. Although this can produce a satisfactory joint, a discovery has been made that it would be desirable to provide an improved method and apparatus for making a welded joint in overlapping strap portions wherein the strap portions are not pressed together as the melted strap layers cool and solidify. This has an advantage, in friction-fusion welding, that the cooling and solidifying strap portions will not be disturbed by the vibrating member as its vibration amplitude is damped to zero upon termination of the welding step.
One related aspect of the present invention is the discovery that, with certain types of strap, the above-discussed higher strength welded joint can be made under the above-described conditions wherein, inter alia, the strap portions are not pressed together after the interface layers have melted and wherein the melted layers are permitted to cool and solidify in the absence of such pressure.